1. Field of the Invention
The present invention relates to a flat-panel X-ray detector, particularly to a TFT-PIN array substrate and an assembly structure for a flat-panel X-ray detector, which are suitable to a UV curable liquid optically clear adhesive (LOCA) process.
2. Description of the Related Art
FPXD (Flat-Panel X-ray Detector) is an important element in the digital medical image technology. FPXD, with a larger field of detection area and higher detective quantum efficiency (DQE), provides excellent image quality of radiography and simplifies the work flow of diagnostic. TFT-FPXD comprises a scintillator substrate and a TFT-PIN array substrate. The TFT-PIN substrate further comprises a TFT (Thin Film Transistor) and a PIN (Positive-Intrinsic-Negative) photodiode. There are various types of scintillators, but the CsI:Tl (Tl-doped CsI) scintillator is a frequently-used one. CsI:Tl is deposited on an aluminum substrate or a graphite-fiber substrate by a thermal evaporator. The crystal of CsI:Tl has a polycrystalline structure or a column structure. The polycrystalline structure has an advantage of simple fabrication parameters and a disadvantage of severe image blur caused by a larger scatter angle. An X-ray beam excites a scintillator to emit visible light. A larger scatter angle of the visible light emitted by the scintillator may cause the neighboring pixels to operate falsely, decrease the spatial resolution, and lower the detective quantum efficiency. The column structure has disadvantages of complicated fabrication parameters of single crystal and low stability of the yield, especially in a large-area FPXD. However, the column structure has an advantage of a smaller scatter angle, which can decrease noise and increase image resolution.
Below are introduced the conventional assembly structures of a scintillator substrate and a TFT-PIN array substrate. Refer to FIG. 1. PSA (Pressure Sensitive Adhesive) 400 is arranged between a scintillator substrate 410 and a TFT-PIN array substrate 420, and they are assembled together via pressing them. The PSA-based assembly method is more suitable to assemble a scintillator 412 having a flexible substrate 411 to the TFT-PIN array substrate 420. Refer to FIG. 2. Ambient temperature-humidity cured or thermoset LOCA (Liquid Optically Clear Adhesive) 430 is arranged between a scintillator substrate 410 and a TFT-PIN array substrate 420, and they are assembled together via pressing them. The LOCA-based assembly method is more suitable to assemble a scintillator 412 having a rigid substrate 411, which is made of a hard material (such as aluminum or graphite fiber), to the TFT-PIN array substrate 420. The LOCA-based assembly method has a disadvantage of long curing time, which to likely to affect yield and alignment precision. Refer to FIG. 3. There is also an assembly method, wherein external pressure is applied to fix a scintillator substrate 410 and a TFT-PIN array substrate 420. However, the method is likely to generate gaps between the scintillator substrate 410 and the TFT-PIN array substrate 420. The difference between the refractivities of the air in the gaps and the scintillator 412 would decrease light transmittance and thus causes variation of light transmittance and unevenness of brightness.
Refer to FIG. 5A and FIG. 6 respectively a diagram showing a conventional TFT-PIN array substrate and a sectional view taken along Section Line A-A in FIG. 5A. The PIN photodiode 423 of the TFT-PIN array substrate 420 includes a metal layer 431, a photoelectric conduction layer 433, a transparent conduction layer 434, and a common wiring 460. The common wiring 460 is electrically connected with the transparent conduction layer 434 of the PIN photodiode 423 through a contact hole 451. The conventional PIN photodiode 423 of the TFT-PIN array substrate 420 has a metal layer 431 spread on the whole surface. Thus, the long-wavelength ultraviolet ray having a wavelength of 365 nm (UVA) cannot penetrate metal layer 431. Consequently, UV curable LOCA is hard to cure in the conventional PIN photodiode 423 of the TFT-PIN array substrate 420. Therefore, UV curable LOCA cannot apply to the fabrication of the conventional assembly structure of flat-panel X-ray detectors.